Method of forming a multilayer printed circuit board and product thereof

ABSTRACT

In a process for fabricating a multilayer printed circuit board with permanent innerlayers of photoresist, the photo-imageable composition used to form the hard permanent layer comprises a polymerizable acrylate monomer; an oligomer formed by the reaction of an epoxy resin and an acrylic or methacrylic acid; a photosensitive, free radical generating initiator for polymerization of the acrylic monomer and oligomer; a curable epoxy resin; a curing agent for the epoxy resin; and, optionally, a crosslinking agent reactive with hydroxyl groups. After exposure and development of a layer of the photoimageable composition and etching of the underlying metal layer on a board, the resultant photoresist is left on the circuitry traces. The innerlayers of photoresist are stacked in a press where it initially conforms under heat and pressure to fill voids between the innerlayers and then cures to permanence.

This is a continuation of application Ser. No. 08/749,042 filed on Nov.14, 1996, now abandoned; which is a continuation of 08/662,816 filedJun. 12, 1996, now abandoned; which is a continuation of 08/446,391filed May 22, 1995 now abandoned; which is a division of 08/271,614filed Jul. 07, 1994, now abandoned; which is a C-I-P of 07/883,436 filedMay 15, 1992, now abandoned.

The present invention is directed to a method for forming a multilayerprinted circuit board, and more particularly to a method of forming amultilayer printed circuit board in which a photoimageable compositionused to print the individual innerlayers forms substantially all of thedielectric material separating the innerlayers from each other.

BACKGROUND OF THE INVENTION

Multilayer printed circuit boards comprise a stack of individual printedcircuit boards or innerlayers separated by dielectrical material. Thecircuitry of the several innerlayers is electrically connected by boredand plated- through holes. Multilayer printed circuit boards providecircuitry in a three-dimensional array and are therefore advantageouslyspace-saving, relative to individual printed circuit boards, whichprovide at most two layers of circuitry on a two-sided board.

These printed circuit boards are commonly provided with internal groundand power planes. These internal planes are frequently solid sheets ofcopper interrupted only by clearance holes (the perforations requiredfor electrically isolating the through hole pattern of the printedcircuit board). Ground and power planes provide power voltage andcurrent and ground connections for the components of the multilayerprinted circuit. A second function of the ground and power planes is toprovide electromagnetic shielding for the multilayer printed circuitboard and reduce the electromagnetic and radio frequency interference.Multiple ground and power planes and additional ground planes on thesurface layers with the conductive pattern are common.

When components are mounted on a multilayer printed circuit board andmass soldered in place at temperatures on the order of about 275° C.(527° F.), a severe thermal shock is applied to the insulating layersplaced between two copper planes, such as the insulating layer betweenan internal ground plate and ground shield on the surface surroundingthe conductor pattern. Frequently, delamination will occur and blisterswill form between the ground shield on the surface and the internalground or power plane. Delamination and blistering have been a problemwith multilayers made by a fully additive, semi-additive, or subtractivesequential processes.

In a conventional subtractive procedure for forming multilayer printedcircuit boards, the innerlayers are formed in the manner of individualtwo-sided printed circuit boards. A blank board formed of epoxy that iscovered on each side with a metal layer, such as copper, is coated oneach side with a layer of primary photoresist. The primary photoresistmay be coated as a liquid resist or applied as a dry film. The layers ofphotoresist are exposed to patterned actinic radiation by placingartwork over the photoresist layers and directing light through theartwork to the photoresist layers. Subsequently, the exposed photoresistlayers are developed in an appropriate aqueous medium to remove exposedor unexposed portions of the photoresist layers, depending upon whetherthe photoresist is positive-acting or negative-acting. The metal layeris then etched away in the areas from which the overlying photoresisthas been removed. After etching, the remaining portions of thephotoresist layers are stripped from the surfaces of the board. In theconventional procedure, the copper surfaces which have been uncovered bythe stripping process are then treated to enhance adhesion of the copperto the pre-preg in subsequent steps.

The individual innerlayers are stacked with alternating layers of"prepreg" material, which typically comprises an epoxy/fiberglasscomposition. The assembly of innerlayers and prepregs is then pressedtogether and heated. The prepreg material initially flows under the heatand pressure, filling the voids between the innerlayers, and eventuallycures under these conditions to form a hard permanent dielectricmaterial separating the individual circuitry layers.

It has been the practice to apply an initial, strongly adherent oxidelayer to the copper to enhance the bond between the copper conductivepatterns and the insulating layers. The oxide layers are usually appliedby immersing the copper surface in hot (40°-110° C.), strongly alkaline,hypochlorite solutions. This immersion produces a black, dendritic oxidelayer with a high surface area for adhering to organic films, coatings,and laminated layers. In the printed circuit industry, this oxide iscommonly called "black oxide".

To electrically connect the circuitry of the several innerlayers, holesare drilled through the cured assembly. The heat so generated causes anepoxy resin "drill smear" on the inner layer copper surface around thedrilled hole prior to the electroless plating of the hole wall. The holewalls are plated to create electrical connections to the inner copperplanes. The desmearing and plating solutions dissolve the black oxidesurrounding the holes and leave non-adherent rings around the holes.This is known as "pink ring" because of the ring of copper visible inthe pattern of black oxide. At the pink ring, the adhesion between thecopper plane and the laminated insulating layer over it is reduced.Ionic contamination and the failure of insulation between holes occurwhere pink ring is found.

As can be seen, the above process requires a number of steps, and it isa primary object of the present invention to produce multilayer printedcircuit boards utilizing fewer production steps.

It is a related object of the invention to eliminate the need for anoxide treatment of the copper layers to improve their adhesion to theepoxy/fiberglass substrate.

It is a further related object of this invention to eliminate pink ringon the innerlayers of a multilayer printed circuit board.

It is yet another related object of this invention to eliminate wedgevoids caused by the action of desmearing chemicals at the blackoxide/epoxy interface on the innerlayers.

It is another object of this invention to provide a multilayered printedcircuit board in which the prepreg dielectric material is substantiallyreplaced by a permanent innerlayer of photoresist material.

It is another object of this invention to provide permanent innerlayerresists which are very thin and can be imaged on contact.

It is a related object to provide permanent innerlayer resists havingenhanced printing resolution capabilities.

It is a further related object to provide multilayer printed circuitboards having improved resolution of the circuitry images thereon.

Other objects of the invention include reducing the materials used inthe process of producing multilayer circuit boards and limiting thenegative impact on the environment associated with prior compositionsand methods for making such boards.

SUMMARY OF THE INVENTION

In accordance with the present invention, the boards to be used asinnerlayers of a multilayer printed circuit board are formed using aresist that is not only photoimageable but is hardenable to form apermanent dielectric material. The individual innerlayers are printed ina usual manner. A board of dielectic base material, such as a glassfiber reinforced epoxy resin, having a layer of copper on each sidethereof is covered on both sides with a hardenable photoimageableresist. The resist may be applied by a variety of methods such as screenprinting, electrostatic spray coating, spin coating, curtain coating,single- or double-sided roller coating, dip coating, extrusion coating,or as a layer or laminate of dry film. The resist is then exposed topatterned actinic radiation and developed in an appropriate developer toremove either the exposed or non-exposed portions of the photoresist.Next, the processed board is subjected to an alkaline etchy or an etchwith an acidic solution of cupric/cuprous chloride that removes themetal layers from those regions where the resist has been removed. Theresist, rather than being stripped after exposure, development andetching, is left on the innerlayers as a supplement for or replacementof the prepregs normally used to form dielectric layers between theinnerlayers. In the manner of prepregs, the photoimageable compositionlayer, under heat and pressure, initially conforms to the contours ofthe innerlayers, filling in the voids between the innerlayers, andsubsequently cures to form at least a portion of permanent dielectriclayers separating the circuitry of the individual innerlayers. Themultilayer board is completed in a normal manner by boring holes throughthe cured assembly, removing the "drill smear" from the copper layers,and lining the bored holes with metal by an electroless platingprocedure.

The present invention provides an improved process for fabricating amultilayer printed circuit board, rigid or flexible, which comprises:

(A) providing a plurality of innerlayer boards, each comprising adielectric base and a metal layer thereon;

(B) coating each of the metal layers with a layer of a photoimageablecomposition comprising:

(a) from about 5 to about 40% by weight of a polymerizable acrylatemonomer;

(b) from about 5% to about 35% by weight of an oligomer formed by thereaction of an epoxy resin and an acrylic or methacrylic acid;;

(c) a photosensitive, free radical generating initiator forpolymerization of the acrylate monomer and the oligomer;

(d) a curable epoxy resin;

(e) a curing agent for the epoxy resin; and

(f) from 0% to about 15% by weight of a cross-linking agent reactivewith hydroxyl groups, all based on the total weight of components (a)through (f);

(C) exposing each of said layers of photoimageable composition to apattern of actinic radiation to polymerize the acrylic monomer andoligomer;

(D) creating photoimaged layers of the composition on the boards bydissolving the unexposed portions of each layer in an aqueous alkalinemedium to uncover metal according to the pattern;

(E) etching the uncovered metal to remove it;

(F) stacking the boards having the photoimaged layers, with or withoutstandard prepreg material, and applying heat and pressure to the stackto cure the composition in each layer and form hard, permanentinnerlayers between the boards; and

(G) connecting the circuitry of the stacked boards.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation of an assembly of exposed, developed and etchedinnerlayers of this invention between two outerlayers as they are aboutto be pressed to form a multilayer printed circuit board in accordancewith the present invention.

FIG. 2 is an elevation of a multilayer board made from the assembly ofFIG. 1.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The particular monomer combinations, monomer/oligomer combinations, andtheir interaction with other components in the formulation will affectthe degree of tackiness and other properties of the photoresist that aredesired. Preferably, the photoimageable composition is prepared in twoparts, the first part containing the oligomer and the second containingthe epoxy resin, and then mixed shortly before application to thesubstrate. The temperature during preparation is chosen to facilitatehigh shear mixing of the components of the two parts; it may beconveniently at room temperature or high as about 200° F. atsuper-atmospheric pressure.

As described above, the photoimageable composition is applied as a layerto the copper clad substrate (or to a support sheet to form a dry filmwhich is then laid on the copper). The particular application methodused will also depend to some degree on the particular formulation ofthe photoimageable composition used. In a preferred roller coatingoperation, there is no differential between the speed of the photoresistapplication rollers and the drying oven conveyor speed so that scoringat the edges of the board does not occur. The conveyor speed may be fromabout 5 to about 7 feet per minute. Suitably, the groove density of therollers is 40 threads per inch, the groove depth is 0.010 inch, and thegroove angle is 76°. After application, the wet layer is dried to removeorganic solvent. During this drying, some reaction of components mayoccur. The thermally driven reaction serves to increase crosslinkdensity and advance the overall molecular weight of the film.Condensation reactions may occur between an anhydride and amelamine-formaldehyde resin, or between an epoxy cresol novolac and ahydroxyl- and/or carboxylic acid functional oligomer.

If there is a significant hold time after drying before the desiredexposure of the photoimageable layer, it may be covered by a sheet ofblack polyethylene or the like. It is then exposed to patterned actinicradiation at an energy level sufficient to cause enough crosslinking toprevent attack by the aqueous alkaline developer that follows in theconventional process. The intensity and time of the light exposure mustalso be correlated with the drying temperatures and the oven conveyorspeed to achieve the optimal performance. It is preferred to effect a UVcure at an energy level of from about 150 to about 350 millijoules/cm²to tie up any unreacted acrylate prior to lamination and preferablyafter etching of the copper exposed by the developer. Additional UVflood irradiation upon development prior to etching of the exposedcopper is not required.

The ability to be developed with aqueous or alkaline aqueous solutionsand with or without organically based antifoam agents, is an importantadvantage of the photoimageable composition of the present invention.Eliminating the need for solvent-based developers eliminates the cost ofthe solvents as well as health, environmental and recycling problems.Although the films formed in accordance with the invention aredevelopable in aqueous solutions without any organic solvents,developers may include some organic solvents, providing that the addedorganic solvent does not solubilize the exposed portions of the film.

In FIG. 1 of the drawings, each innerlayer 10 comprises a base layer 12of dielectric material, circuitry traces 14 on the opposite facesthereof, and the photoimageable composition 16 overlying the circuitrytraces 14. Although not shown here for the sake of clarity, theouterlayers 18 may have circuitry traces on their inner faces, only,before pressing of the assembly; the outer faces thereof would stillretain a bare layer of copper foil which may be coated with resist,exposed, developed, and etched after the multilayered board comes out ofthe press.

In the prior art method of forming a multilayer circuit board, thephotoimageable composition used instead of composition 16 would havebeen stripped from the circuitry traces 14 and the exposed circuitrytraces would have been treated in a chemical bath. In the process of thepresent invention, the photoimageable composition 16 is not stripped.The need for a stripper is eliminated, as are any problems associatedwith disposal of the stripper. Elimination of the stripping stages ofthe conventional process permits increased throughput of a multilayerboard and reduces the resources required for its production.

At this point, there are several options available in forming themultilayer circuit board. In the option shown in FIGS. 1 and 2, theinnerlayers 10 and outerlayers 18 are merely stacked in a standardmultilayer press and the assembly is subjected to heat and pressurewhich initially squeezes the still flowable photoimageable composition16 into the voids between the innerlayers 10 to form the board 20 ofFIG. 2. The board 20 is maintained under heat and pressure for fromabout 3 to about 3.6 hours, during which time the photoimageablecomposition 16 cures to form hardened, permanent dielectric layers 22which electrically separate the circuitry traces 14 of the innerlayers10.

The conditions of heat and pressure are somewhat dependent upon theparticular photoimageable composition selected for the process. In asuitable procedure known as the hot start/dual pressure procedure or the"kiss" procedure, the assembly is held at about 350° F. (177° C.) atabout 50 psi for a short while and then the pressure is increased tofrom about 450 to about 700 psi. In the hot start/full pressureprocedure, the temperature is 350° F. but a pressure of from about 600to about 800 psi (56 kg/cm²) is held for the full 3 to 3.6 hours. Thecured assembly is suitably cooled down at a rate of about 10° F./min. Anoptional postbake of the dielectric layers 22 may be used to furthercure or anneal it.

The option of utilizing only the photoimageable composition to form thedielectric layer 22 has the advantage of eliminating any need forprepregs or additional material, the inclusion of which would constituteadditional effort. On the other hand, if only the photoimageablecomposition 16 is to be used to form the dielectric layers 22, it mustbe applied relatively thickly in order that the portions of thephotoimageable composition 16 remaining after development providesufficient material to not only fill in the voids between the traces 14but also to provide a sufficiently thick dielectric barrier between thetraces of adjacent innerlayers 10. Because the photoimageablecomposition is relatively expensive, thick layers of it are expensive toproduce. Also, a thick photoimageable layer produces poorer resolutionthan a thin one. If the initially applied photoimageable layers are toserve as the sole source of material for the dielectric layer 22, thethickness of the photoimageable layer will vary from about 2.2 mils when1 oz. copper foil (1.4 mils thick) is used to about 3.2 mils when 2 oz.copper foil (2.8 mils thick) is used.

Another option (not illustrated) is to apply additional dielectricprepreg material to the board after etching. Again, the photoimageablecomposition layer is not removed from the traces, avoiding the need forstripper and treatment to avoid oxidation. The additional material couldbe more of the photoimageable composition 16, a compatible curablesystem (without the photosensitive components to minimize cost), or evenconventional prepregs. In any case, the photoimageable composition layerthat is left on the traces forms a portion of the dielectric layers 22in the multilayer board.

The board 20 is then taken out of the press, the outerlayers areprocessed as desired and the resulting multilayer board is thenprocessed in a conventional manner. Holes are bored through the layers,and the hole walls are plated to electrically connect the circuitry ofthe several layers.

The composition 16 according to the present invention includes anacrylic chemical system by which the composition is photo-polymerizedand an epoxy chemical system which is curable to harden the compositionafter exposure and development. The two chemical systems, however, arenot exclusive of each other and components of the two chemical systemsare believed to interact chemically with each other. This is especiallytrue when the composition includes a cross-linking agent (f).

The acrylic system includes the acrylate monomers (a), theepoxy-acrylate oligomer (c) and the photoinitiator (b). The epoxy systemincludes the epoxy resin (d) and the acidic curative (e) therefor. If across-linking agent (f) is used, it is selective to be reactive withfree hydroxyl groups of components of both the acrylic and epoxysystems.

In one embodiment of the invention, the preferred photo-imageablecomposition comprises (a) from about 5 to about 40% of the acrylatemonomers; (b) from about 3 to about 15% photo initiator; (c) from about5 to about 35% of an epoxy-acrylate oligomer; (d) from about 20 to about80% of an epoxy resin; (e) from about 0.1 to about 10% of an acidiccurative for the epoxy resin and; (f) from about 1 to about 15% of thecross-linking agent. The preferred composition has a photo-sensitivechemical system which renders exposed portions of the resist insolublein alkaline aqueous developer and an epoxy-based chemical system whichis curable to provide permanence and hardness to the material subsequentto exposure, development, etching and curing.

A level of the acrylate monomer (a) lower than 5% may be insufficient toinsolubilize the portions of the photoresist layer that have beenexposed to actinic radiation. Levels of acrylate monomer higher than 40%may result in an innerlayer which is too soft. The acrylate monomerpreferably comprises from about 5 to about 35 percent of the totalweight of components (a)-(f). Unless otherwise stated, all percentagesof components (a)-(f) are calculated relative to the total weight ofcomponents (a)-(f). The amounts of any additional ingredients, such asfillers, solvents, etc. are also calculated relative to the sum of theweights of (a)-(f). The dried composition of (a)-(f) is soluble inalkaline aqueous solution, whereby the photoimageable composition layer16 on a circuit laminate is developable with alkaline aqueous solution.

The acyl moiety of the acrylate monomers is represented by the formula:[RCH=CHC=O]_(x) wherein R is hydrogen or alkyl, and x is an integer from1 to 4. The monomers are selected from a variety of esters of acrylicand methacrylic acids, such as methyl acrylate, methyl methacrylate,hydroxyethyl acrylate, butyl methacrylate, octyl acrylate, 2-ethoxyethyl methacrylate, t-butyl acrylate; 1,5-pentanediol diacrylate,N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate;1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethyleneglycol diacrylate; 1,3-propanediol diacrylate, decamethylene glycoldiacrylate, decamethylene glycol dimethacrylate;1,4-cyclohexanedioldiacrylate; 2,2-dimethylol propane diacrylate,glyceroldiacrylate, tripropylene glycol diacrylate, glyceroltriacrylate, trimethylolpropane triacrylate, pentaerythritoltriacrylate; 2,2-di(p-hydroxy-phenyl)-propane diacrylate,pentaerythritol tetracrylate; 2,2-di(p-hydroxyphenyl)-propanedimethacrylate, triethylene glycol diacrylate,polyoxyethyl-2-2-di(p-hydroxyphenyl)-propane dimethacrylate, triethyleneglycol dimethacrylate, polyoxypropyltrimethylolpropane triacrylate,ethylene glycol dimethacrylate, butylene glycol dimethacrylate;1,3-propanediol dimethacrylate; 1,2,4-butanetriol trimethacrylate;2,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritoltrimethacrylate; 1-phenyl-ethylene-1,2-dimethacrylate, pentaerythritoltetramethacrylate, trimethylol propane trimethacrylate; 1,5-pentanedioldimethacrylate; 1,4-benzenediol dimethacrylate, and the triacrylate oftris-(2-hydroxyethyl) isocyanurate. Polyfunctional acrylates andmethacrylates, i.e., those having three or more acrylic or methacrylicmoieties, are preferred because of the increased photospeed gained bytheir presence.

Epoxy-acrylate oligomer (c) means herein an oligomer formed from anepoxy backbone which is reacted with acrylic or methacrylic acid, a lowmolecular weight polyacrylic or polymethacrylic acid, or a mixture ofthem, so that at least about 90% of the epoxy groups are esterified withthe acid. In the reaction of the acid with the epoxy groups of the epoxyresin, the acid molecules each form an ester bond with the resin and ahydroxyl group is formed on the vicinal carbon atom. Becausesubstantially all of the epoxy groups are reacted with the acrylic ormethacrylic acid moiety, the oligomer functions primarily as an acrylateor methacrylate, said moieties of the oligomer polymerizing along withthe acrylic monomers during the photoinitiated reaction which rendersexposed portions of the photoimageable composition layer insoluble toaqueous alkaline solution. The substantial hydroxyl functionalityprovides the basis for cross-linking with the cross-linking agent (f),if such a cross-linking agent (f) is used.

Preferred epoxy-acrylate oligomers are diacrylate (or methacrylate)esters of bisphenol A type resins, novolac resins, and mixtures thereofhaving a maximum epoxide content of about 0.5% by weight.

A diacrylate ester of a bisphenol A epoxy resin may be the soleepoxy/acrylate oliogomer. These oligomers are developed to combine goodUV/EB cure response along with the chemical resistance and durability ofthe epoxy resins. Said oligomers are derived from bisphenol A resinshaving a functionality of two so they also have a functionality of two.

The synthesis of polyfunctional acrylate resins and other unsaturatedesters from the corresponding epoxy derivatives is described in thefollowing U.S. Pat. Nos.: 3,256,226; 3,317,465; 3,345,401; 3,373,221;3,377,406; 3,432,478; 3,548,030; 3,564,074; 3,634,542 and 3,637,618; allof which are incorporated herein by reference.

The epoxy-acrylate oligomer preferably comprises from about 5 to about35% of the total weight of components (a)-(f), and more preferably fromabout 12 to 35%. Epoxy-acrylate oligomers used in the photoimageablecomposition preferably have molecular weights of from about 500 to about2000.

Also required in conjunction with the polymerizable acrylate substanceis a chemical initiator system which generates free radicals in thepresence of actinic radiation and thereby causes the polymerization ofthe acrylic substances. Polymerization of acrylic monomers and acrylicmoieties of the epoxy-acrylate oligomers into a three dimensionalstructure insolubilizes the photoimageable composition. The choice ofthe photosensitive, free radical generating initiator system is notconsidered to be critical to the practice of the present invention, anda wide variety of such compounds may be successfully utilized in thepractice of this invention. Examples of chemical photoinitiator systemsinclude benzophenone, benzoin ether, benzil ketals, ispropylthioxanthoneand acetophenone and its derivatives. Other suitable initiator systemsare described, for example, in U.S. Pat. Nos. 3,469,982, 4,451,523 and4,358,477, the teachings of which are incorporated herein by reference.The amount of photoinitiator employed may vary over a wide range,depending upon the polymerizable acrylic substances, the particularphotoinitiator system and the desired time of development. Generally,the photoinitiator chemical system comprises from about 3 to about 15%of the total weight of components (a)-(f).

The material which imparts the excellent hardness and durability to thefilm or layer after development and final curing is the epoxy resin ormixture of epoxy resins. The epoxy resin or resins comprise from about20 to about 80% (preferably from about 30 to about 60%) of the totalweight of components (a)-(f). At high temperatures and/or in thepresence of a catalyst, the epoxy groups of the resin molecules open andreact with other materials present. Primarily, the epoxy resin moleculesreact with the acidic curative (e); however, to a certain extent, theepoxy molecules react during final curing with the cross-linking agent(f), and perhaps also with the photo-polymerized acrylic material andany remaining unpolymerized acrylate monomers or moieties. Preferably,the epoxy resin or mixture of resins is solid at about room temperature.The photoimageable composition can be applied as a liquid film to thesubstrate after dissolving the solid epoxy resin in the solvent-basedmixture first.

A wide variety of epoxy resins is suitable for use in accordance withthe present invention. Typically, epoxies of the Bisphenol A and novolactype are used. Other suitable epoxy resins are described, for example,in U.S. Pat. No. 4,092,443, the teachings of which are incorporatedherein by reference. Cycloaliphatic epoxides, such as those sold underthe trade names Cyanacure UVR-6100 and UVR-6110 by Union Carbide,Danbury, Conn. are also useful. Epoxy resins used in accordance with theinvention preferably have epoxide equivalents of from about 90 to about700.

The photoimageable composition according to this invention is intendedto be hardened to form a permanent innerlayer. Hardening is primarilyattributed to the curing of the epoxy resin. To promote sufficientlyrapid curing of the epoxy resin, the photoimageable composition of thepresent invention employs an acidic curative. Acidic cure catalystsinclude not only substances which have free carboxyl groups, but alsochemicals such as anhydrides, which may produce free carboxyl groups.For many applications of the invention, an anhydride, e.g., an anhydrideof a multifunctional carboxylic acid is the preferred curative. Otheruseful catalysts are those having a blocked carboxylic group, whichbecomes deblocked at a threshold temperature. Epoxy cure catalysts aregenerally used at levels of from about 0.1 to about 10% of the totalweight of components (a)-(f).

Although photoimageable compositions in accordance with the presentinvention do not necessarily require an additional cross-linking agent,a cross-linking agent (f) is highly desirable. A cross-linking agent isparticularly useful in connecting the acrylate chemical system and theepoxy chemical system in a single interconnected network in theinnerlayer. Free hydroxyl groups on the epoxy resins and the epoxy-acrylate oligomers, generally provide the basis for such cross-linking.The cross-linking agent (f) is typically used at a level of at leastabout 1%, e.g., from about 2 to about 15%, preferably 1.5-15%.

In accordance with one aspect of the invention, the cross-linking agentis a melamine-formaldehyde resin. During initial application of thephotoimageable composition as a layer, it is believed that themelamine-formaldehyde resin reacts to some extent with the anhydride.This opens the anhydride, providing carboxyl functionality forsubsequent epoxy curing. This initial reaction also has a surface dryingeffect. Melamine/formal-dehyde resin acts to cross-link through freehydroxyl groups. The methylol groups of melamine/formaldehyde resins maybe alkoxylated to give a suitable cross-linking agent. In accordancewith another aspect of the invention, a blocked, multifunctionalisocyanate may be used as the cross-linking agent. The blockedisocyanate is selected to deblock generally at the cure temperature ofthe epoxy resin. An example of a suitable blocked isocyanate isE-caprolactam-blocked isophorone. If a blocked isocyanate is thecross-linking agent and an anhydride is the curative, some preheating ofthe photoimageable composition at a time prior to cure is desirable.Such preheating opens anhydride species, providing the acidfunctionality needed to promote curing of the epoxy resin and neededalso for alkaline development.

The cross-linking agent may also be cyanoguanidine (also known asdicyandiamide or dicyanamide) or a substituted amino triazine such as2-[β-(2'-methylimidazolyl-1']-ethyl-4,5-diamino-s-triazine, which issold under the trademark CUREZOL 2-Mz-Azine. A mixture of thedicyandiamide and substituted triazine may also be used.

A photoresist composition having the following formulation is suitablefor the purposes of this invention:

(a) from about 6.7 to about 7.1% by weight of trimethylol propanetriacrylate;

(b) from about 32.4 to about 34.3% by weight of a diacrylate ester of abisphenol A epoxy resin;

(c) a photosensitive, free radical generating initiator;

(d) from about 35.5 to about 37.6% of an epoxy cresol novolac resin; and

(e) a curing agent for the epoxy resin.

The components of the photoimageable composition are selected to besoluble in a common solvent to form a single-phase liquid composition.As mentioned above, the liquid photoimageable composition may be appliedto a substrate by a variety of application methods. Each applicationmethod has its own peculiarities, and photoimageable compositions inaccordance with this invention may be formulated in accordance with theparticular requirements of the particular method of application.

Components (a)-(f) are selected such that a dried composition of theseis soluble in alkaline aqueous solution, whereby a layer of thephotoimageable composition may be developed in alkaline aqueoussolution, e.g., 1% sodium carbonate.

A mixture of components (a)-(f) is typically too viscous to be easilyapplied as a layer; accordingly, it is generally the practice to dilutethe components (a)-(f) with an organic solvent. Typically, solvent isused at a level of 10-60%, but this will vary depending upon the meansof application. For screen printing, solvent is generally used at10-20%; for electrostatic spray coating, at 20-60%; for curtain coatinggenerally at 40-50%; and for application as a dry film typically about50%. Suitable solvents include, but are not limited to ethylene glycolmonoethyl ether, ethylene glycol monopropyl ether, ethylene glycolmonobutyl ether, ethylene glycol-2-ethylhexyl ether, ethylene glycolmonohexyl ether, diethylene glycol monomethyl ether, diethylene glycolmonoethyl ether, diethylene glycol monopropyl ether, diethylene glycolmonobutyl ether, dipropylene glycol methyl ether, propylene glycolmonobutyl ether, propylene glycol monopropyl ether, propylene glycolmethyl ether, ethylene glycol monoethyl ether acetate, ethylene glycolmonobutyl ether acetate, diethylene glycol monoethyl ether acetate,diethylene glycol monobutyl ether acetate, propylene glycol monomethylether acetate, ethylene glycol diacetate, 2-ethylhexyl acetate, n-butylacetate, isobutyl acetate, n-propyl acetate, ethyl acetate, diacetonealcohol, dimethyl formamide, isophorone, diisobutyl ketone,cyclohexanone, isobutyl isobutyrate, methyl n-amyl ketone, amyl acetate,methyl amyl acetone, methyl isoamyl ketone, 2-nitropropane, methylisobutyl ketone, methyl n-propyl ketone, isopropyl acetate, methyl ethylketone, tetrahydrofuran, acetone, methyl acetate, N-methyl pyrolidoneand butyrolactone.

In addition to the components described above which are essential to aphotoimageable composition of this embodiment the invention, thephotoimageable composition may optionally contain additional componentswhich are standard in the art. The photoimageable composition optionallymay contain organic or inorganic fillers at levels up to about 4%. Someexamples of fillers are micro talc, ground polyethylene, clay, fumedsilica and polyvinylidene fluoride. Also, relatively small amounts offlow-control agents, dyes, antioxidants, etc. may be added. Fillers mayeffect the final appearance of innerlayers, e.g. provide a matte finish.

The photoimageable compositions will now be described in greater detailby way of specific examples.

EXAMPLE 1

A composition was prepared as follows:

    ______________________________________                                        Component*              % Wt.                                                 ______________________________________                                        Tris(2-hydroxyethyl) isocyanurate triacrylate.sup.1                                                   18.7                                                  Methylolated melamine.sup.2                                                                           3.4                                                   Hetron 912 (epoxy methacrylate resin).sup.3                                                           6.2                                                   Ebecryl 3701 (diacrylate ester of                                                                     6.2                                                   a bisphenol A epoxy resin).sup.4                                              Epoxy cresol novolac resin, epoxy eq. 235.sup.5                                                       23.8                                                  Bisphenol A epoxy resin, epoxy eq. 575-685.sup.6                                                      23.3                                                  5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-                                                              8.6                                                   cyclohexene-1,2-dicarboxylic anhydride.sup.7                                  2,2-dimethoxy-2-phenyl acetophenone.sup.8                                                             1.4                                                   2-methyl-1-[4-(methylthio)phenyl]-2-                                                                  6.2                                                   (4-morpholinyl)-1-propanone.sup.8                                             isopropylthioxanthone.sup.9                                                                           2.3                                                                           100.0                                                 PLUS    Leveling Agents, etc:                                                         Modaflow.sup.10     0.9                                                       Byk 361, 306 (equal portions).sup.11                                                              0.8                                                       Malachite green dye.sup.12                                                                        0.08                                                      Filler (fumed silica).sup.13                                                                      1.5                                                       Inhibitor (MEHQ).sup.14                                                                           0.1                                               Solvent     (Amount and type appropriate                                                  to the method of application)                                     Solvent used: Ethyl-3-ethoxy propionate (EEP).sup.15                          ______________________________________                                         *Source: 1. Hitachi Corp.; 2. American Cyanamide; 3. Ashland Chemical; 4.     Radcure; 5. CibaGeigy; 6. Dow; 7. DIC Americas; 8. CibaGeigy; 9. Ward         Blankensop; 10. Monsanto; 11. Byk Mallinckrodt; 12. Penn; 13. Cabosil EH5     14. Aldrich; 15. Kodak                                                   

The photoimageable composition was applied as a wet film via a curtaincoating process. The following coating conditions were used: 11.1 gm wetcoating material per square foot laminate was applied in order toachieve an approximate dry film thickness of 2.0 to 2.2 mils on laminateand 0.8 to 0.9 mil dry film thickness on the copper circuitry. Coatingspeed was 80 to 90 meters per minute. The working viscosity of thephotoimageable composition during coating was a Zahn Cup No. 5 readingof 22 seconds, 25° C. This was equivalent to approximately 60 wt.percent solids. The photoimageable composition was dried at 90° C. for15 minutes, and cooled to room temperature. The second side was thencoated in a manner identical to the first side. The second side wasdried to a tack-free surface at 90° C. for 30 minutes. Diazo artwork wasplaced directly on the film, and the film was exposed to actinicradiation having a UV energy exposure level of at least 350 mjoules/cm²at the working surface. Exposure time should be such that the exposedportion of the coating remains intact during the aqueous developingprocess. The film was developed in a basic aqueous solution, i.e., 1% byweight sodium carbonate mono-hydrate. The film was UV-cured by exposureto actinic radiation having a UV energy exposure level of at least 2joules/cm². The exposed copper was then etched in a cupric chloridesolution (an ammoniacal etchant is an alternative) to generate thecircuitry traces. The etched circuit laminates with the innerlayerresist intact are then pressed into a multi-layer board by the "kiss"procedure described above.

The performance of rigid multilayer printed circuit boards fabricatedwith a permanent inner layer photo resist (PILPR) according to thegeneral procedure of Example 1 was measured by test procedures requiredby the IPC-ML-950-C Performance Specification and MIL-P-55110 GeneralSpecification for Printed Wiring Boards. Test coupons of these boardswere made to have nominal thicknesses of 0.092, 0.125, and 0.225 inch.The IPC-A-43 comb pattern was imparted to some of the coupons andfour-wire plated through hole (PTH) coupons were also tested. Ingeneral, it was determined that such boards will meet or exceed therequirements of each specification. Buried layer insulation resistancemeasurements and high voltage breakdown measurements are conducted onthe IPC-A-43 coupons. The high voltage measurements provide a means fordetecting delamination of the innerlayers. The PTH test coupon allowsmeasurement of the PTH barrel and interface resistance so that crackinitiation and propagation can be detected.

In particular, the boards of this invention meet the standards of thethermal shock test of MIL-P-55110, para. 4.8.6.3, type GP and thethermal stress (solder float) test of IPC-ML-950-C, Class 3,para.4.6.5.1. Said boards exceed the Ionic Contamination requirements ofNorthern Telecom Corporate Standard 5040.02, Table 5.01 when tested bythe method described in Diceon Std. Doc. #108384-026 (DiceonElectronics, Inc.) which is equivalent to methods 2.3.38 and 2.3.39 ofIPC-TM-650. A maximum extraction of 6.4 μg/in² is allowed by thosestandards and the average value for six boards of this invention was 2.7μg/in².

EXAMPLE 2

A composition was prepared as follows:

    ______________________________________                                        Component               % Wt.                                                 ______________________________________                                        Tris(2-hydroxyethyl) isocyanurate triacrylate                                                         18.5                                                  Methylolated melamine   3.4                                                   Hetron 912              6.1                                                   Novacure 3701           6.1                                                   Epoxy cresol novolac resin, epoxy eq. 235                                                             46.7                                                  5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-                                                              8.5                                                   cyclohexene-1,2-dicarboxylic anhydride                                        2,2-dimethoxy-2-phenyl acetophenone                                                                   3.7                                                   2-methyl-1-[4-(methylthio)phenyl]-2-                                                                  4.7                                                   (4-morpholinyl)-1-propanone                                                   isopropylthioxanthone   2.2                                                                           99.9                                                  PLUS    Leveling Agents, etc:                                                         Byk 361, 306 (equal portions)                                                                     0.8                                                       Pigment (Penn Chips)                                                                              1.0                                                       Filler (precipitated silica, Syloid 72)                                                           1.5                                                       Inhibitor (MEHQ)    0.1                                               Solvent     (Amount and type appropriate                                                  to the method of application)                                     Solvent used: Ethyl-3-ethoxy propionate                                       ______________________________________                                    

EXAMPLE 3

Each of the compositions of Examples 1 and 2 were applied as a wet filmvia an electrostatic spray process as follows:

The photoimageable composition was diluted to a Zahn cup No. 2 viscosityof 60 seconds using an appropriate solvent. Both sides of the baselaminates were coated as they were conveyed at 5 feet per minute throughan aerosol spray of the solution charged via the application of 70,000volts. Drying of the laminates took place at 80° C. for 30 minutes.Diazo artwork was placed directly on the tack-free film and the film wasexposed to actinic radiation having a UV energy exposure level of about150 to about 400 mills joules/cm² at the working surface. Exposure timewas such that the exposed portion of the coating remained intact duringthe development of the film in a basic aqueous solution of 1% by weightof sodium carbonate monohydrate. The film was UV-cured by exposure toactinic radiation having a UV energy exposure level of from about 1 toabout 4 joules per cm². The exposed copper was then etched in a cupricchloride solution to generate the circuitry traces. The etched circuitlaminates with the innerlayer resist intact are then pressed into amulti-layer board by the "kiss" procedure described above

EXAMPLE 4

Each of the compositions of Examples 1 and 2 were applied as a wet filmto base sheets so as to form dry films as follows. A wet film of 6 milthickness was applied by draw down to a base sheet. The solventevaporated from the film which was then covered with a polyester sheet.The film was applied to a circuit board laminate by hot roll lamination.The diazo artwork was placed on the cover sheet and the film was exposedto actinic radiation having a UV energy exposure level of at least 350mjoules/cm² at the working surface. Exposure time was such that theexposed portion of the coating remained intact during the aqueousdeveloping process. The cover sheet was removed from the film beforedeveloping. The film was developed in a basic aqueous solution, i.e., 1%by weight sodium carbonate monohydrate. The film was UV-cured byexposure to actinic radiation having a UV energy exposure level of 2joules/cm². The exposed copper was then etched in a cupric chloridesolution to generate the circuitry traces. The etched circuit laminateswith the innerlayer resist intact are then pressed into a multi-layerboard by the "kiss" procedure described above.

EXAMPLE 5

A composition was prepared as follows:

    ______________________________________                                        Component               % Wt.                                                 ______________________________________                                        Tris(2-hydroxyethyl) isocyanurate triacrylate                                                         7.8                                                   Pentaerythritol Tetraacrylate (Sartomer)                                                              5.7                                                   Epoxy cresol novolac resin, epoxy eq. 235                                                             37.6                                                  Radcure 3701            19.9                                                  E-Caprolactam-blocked isophorone                                                                      9.7                                                   (Diisocyanate based adduct)                                                   5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-                                                              10.3                                                  cyclohexene-1,2-dicarboxylic anhydride                                        2,2-dimethoxy-2-phenyl acetophenone                                                                   5.0                                                   2-methyl-1-[4-(methylthio)phenol]-2-                                                                  2.8                                                   (4-morpholinyl)-1-propanone                                                   isopropylthioxanthone   1.1                                                                           99.9                                                  PLUS    Leveling Agents, etc:                                                         Bubble Breaker (Witco)                                                                            1.9                                                       Byk 077             2.0                                                       Byk 306             1.1                                                       Pigment (Penn Green)                                                                              1.2                                                       Filler (fumed silica, Cabosil EH5)                                                                3.5                                                       Inhibitor (MEHQ)    0.05                                              Solvent     (Amount and type appropriate                                                  to the method of application)                                     Solvent used:  Ethyl-3-ethoxy propionate,                                                    carbitol acetate (1:1).                                        ______________________________________                                    

This photoimageable composition was applied as a wet film via screenprinting on a printed circuit board. The screen mesh was varied from 61to 120 mesh. The screened photoimageable composition was dried at 80° C.for 30 minutes, providing a non-tacky film. A diazo artwork was placeddirectly on the film, and the film was exposed to actinic radiationhaving a UV energy exposure level of 350 mjoules/cm². The film wasdeveloped in a basic aqueous solution, i.e., 1% by weight sodiumcarbonate monohydrate. The film was UV-cured by exposure to actinicradiation having a UV exposure level of 2 joules/cm².

The exposed copper was then etched in a cupric chloride solution togenerate the circuitry traces. The etched circuit laminates with theinnerlayer resist intact are then pressed into a multi-layer board bythe "kiss" procedure described above.

EXAMPLES 6-8

The photoimageable composition described in Example 5 may also be usedin the making of a multi-layered printed circuit board according to thegeneral procedures described in Examples 1 and 3-5.

EXAMPLE 9

A composition of this invention was prepared in two parts, thecomponents of which are a follows:

    ______________________________________                                        Component              % Wt.                                                  ______________________________________                                        PART A                                                                        Novacure 3701 (diacrylate ester of                                                                   48.01                                                  a bisphenol A epoxy resin)                                                    5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-                                                             9.45                                                   cyclohexene-1,2-dicarboxylic anhydride                                        QITX isopropylthioxanthone                                                                           2.59                                                   Irgacure 907           7.35                                                   Cyanoguanidine (CG 1400)                                                                             2.69                                                   BYK 306                1.34                                                   BYK 361                0.60                                                   Malachite green        0.30                                                   Ethyl-3-ethoxy propionate                                                                            27.65                                                                         100.00                                                 PART B                                                                        Epoxy cresol novolac resin, ECN-1299                                                                 52.56                                                  Trimethylolpropane triacrylate                                                                       10.03                                                  2-MZ-Azine (CUREZOL)   0.40                                                   BYK-306                1.54                                                   Cab-O-Sil M5A silica   6.96                                                   Modaflow               0.20                                                   Ethyl-3-ethoxy propionate                                                                            28.32                                                                         100.00                                                 ______________________________________                                    

For the preparation of Part A, the first two components were mixed withthe Malachite green in a portion of the propionate solvent for fourhours and controlling the temperature by regulating the speed of themixer and the flow of cooling water to the reactor jacket. The remainderof the solvent and the remaining components were added while the mixturewas stirred at high shear. After a two hour mixing period, the productwas filtered through a 10 μ bag filter and the filtrate was a dark greenviscous paste having a solids content of 76.4% (w/w), a Hegman grind of7.5, and a Brookfield viscosity of 14,000 cps using a No. 7 spindle at10 rpm for 2 minutes at 25° C.

For the preparation of Part B, a portion of the solvent and the epoxynovolac resin were mixed at high shear for two hours. The flow agents,the triacrylate and the Curezol were added and high shear mixing for 90minutes followed while controlling the temperature in the same manner.The silica was then added over a one hour period, the remainder of thesolvent was added, and the mixture was stirred for another four hours.This part was an opaque tan colored liquid having thixotropicproperties. The viscosity was 3600 cps by the same method as for Part A.The solids content was 69.8% (w/w), and the Hegman grind was 4.5. Equalparts by weight of Part A and Part B were mixed to obtain a photoresistof this invention containing about 5% of the acrylate monomer, about 24%of the epoxy/acrylate oligomer, about 26% of the epoxy novolac resin,and about 28% solvent, by weight. On a solids basis, therefore, theamount of acrylate monomer is about 6.9%, the oligomer is about 33.3%,and the epoxy novolac resin is about 36.1% of the total weight.Laminates were coated with the photoresist using a Burkle double sidedroller coater having a zero speed differential between the applicationrollers and the oven conveyor belt, which was traveling at about 7feet/minute. The wet films were dried in two consecutive temperaturezones, the first zone being at 105° C. and the second at 177° C. The dryfilms manifested little or no tack. The average thickness of the fourfilms was 0.6±0.1 mil. Diazo artwork was placed directly on the surfaceof the films and each was exposed to actinic radiation having apredominant wavelength of 365 nm and an exposure energy level averaging200±25 mj/cm² The step density in each case was 5 or greater. There wasno transfer of the coating from the laminates to the artwork. The 50%breakpoint averaged 30±10 seconds so the developing time in a 1±0.05%aqueous solution of sodium carbonate mono-hydrate at 85° F. was about60±20 seconds.

After drying, the exposed metal is dissolved by drawing the boardsthrough a 3 Normal aqueous solution of cupric chloride, hydrochloricacid, and chlorine at 120° F. at a rate of about 30 inches per minuteand they are then rinsed and dried. The etched circuit-bearing laminatesare then stacked and pressed into a multi-layer board by the "kiss"procedure. Holes are then bored through the multiple layers so that thecircuitries of each layer may be connected later by plating the holewalls. The "drill smear" on the inner copper layers must first becleaned off, however, in a desmearing step by immersing the board in amixture of butyl cellosolve and monoethanolamine at 130° F. for 5minutes, rinsing it with water and then immersing it for five minutes ina mixture of two solutions at 160° F., one solution being 1 molar insodium hydroxide and the other 1 molar in sodium permanganate. The boardis rinsed with water again before being neutralized by immersion in amixture of citric acid and sulfuric acid solutions and then dried.

The holes are then plated through first by an electroless platingprocedure wherein it is immersed in an aqueous solution of sodiumpersulfate for a short time at room temperature, then rinsed and treatedwith an aqueous solution of sodium chloride and hydrochloric acid beforebeing immersed in a catalyst bath of palladium chloride in water at 100°F. for 3-5 minutes. The board is rinsed again and treated with anaqueous solution of stannous fluoborate to fix the palladium catalyst tothe non-conductive surfaces of the board. Again, the board is rinsed andthen immersed in an aqueous solution of copper sulfate, a metal chelatorsuch as sodium tartrate, and EDTA at 100° F. for 20 minutes. The boardis then rinsed and treated with a solution of acetic acid andbenzotriazole to inhibit oxidation and dried. In the second plating stepthe copper plated walls of the holes are given a thicker layer of copperin an electroplating process wherein the thin copper lining of the holesacts as the cathode as the copper ions in a bath of dilute sulfuric acidand copper sulfate are reduced and are replenished by a copper anode.

While the invention has been described in terms of certain preferredembodiments, modifications obvious to one with ordinary skill in the artmay be made without departing from the scope of the present invention.

What is claimed is:
 1. A photoresist composition consisting essentiallyof(a) from about 6.7 to about 7.1% by weight of trimethylolpropanetriacrylate; (b) from about 32.4 to about 34.3% by weight of anepoxy/acrylate oligomer consisting of a diacrylate ester of a bisphenolA epoxy resin; (c) a photosensitive, free radical generating initiator;(d) from about 35.5 to about 37.6% of an epoxy cresol novolac resin; (e)a curing agent for the epoxy resin; (f) from 0% to about 15% by weightof a cross-linking agent reactive with hydroxyl groups, all based on thetotal weight of components (a) through (f); and (g) up to about 4% byweight of a filler.
 2. The composition of claim 1 wherein thecross-linking agent is a mixture of cyanoguanidine and a substitutedamino triazine.
 3. The composition of claim 2 wherein the substitutedamino triazine is2-[β-{2'-methylimidazolyl-1'}]-ethyl-4,5-diamino-s-triazine [is mixedwith the cyanoguanidine].
 4. The composition of claim 1 wherein theinitiator is from about 3 to about 15% and the cross-linking agent isfrom about 1 to about 5% by weight of the photoresist.